Field of the Invention
The present invention relates to a fuel control apparatus for an engine and, more particularly, to a fuel control apparatus to allow an air-fuel ratio in a gas mixture to be controlled by switching between an open loop control and a feedback control in response to the operating state of the engine and further to allow a control by learning or a learning control to be conducted at the time of the feedback control.
In an engine, particularly an internal combustion engine for vehicles, an air-fuel ratio is frequently controlled according to an output from an air-fuel ratio sensor such as an O.sub.2 sensor, that is, a fuel amount supplied to the engine is frequently controlled (or corrected) so that the air-fuel ratio of a mixture gas become a target value.
This feedback control has a problem in the responsiveness of the control. Thus, control by learning or a learning control, in addition to the feedback control, has been recently proposed. In Japanese Patent Application Laid-Open No. 59335/1983, the feedback correction is conducted using a feedback correction value that is obtained in accordance with an output from an O.sub.2 sensor for detecting the oxygen concentration (air-fuel ratio) in exhaust gas. A learning value is calculated according to the feedback correction value and the learning value is stored in memory means having, for example, a plurality of learning zones divided at every predetermined vehicle speed. At a certain vehicle speed while conducting a sort of prospect control by the learning correction in accordance with the learning value stored in the learning zone of the memory means, corresponding to the vehicle speed, the feedback control as described hereinabove is carried out. Accordingly, an amount of correction by the feedback control (feedback correction value) can be reduced by the amount of the prospect control with the learning value, thus leading to a higher responsiveness of the control.
In particular, according as an increase in the number of learnings as the same driving state is continued for a long period of time, the amount of correction by the feedback correction can be extremely reduced. Also such a learning control may absorb the individual difference of engines, in particular, the individual difference of fuel injection valves, which affects the setting of supplying the fuel amount to a great extent or the individual difference of sensors for detecting the amount of intake air.
In general, the feedback control to control the air-fuel ratio of the gas mixture is frequently used only in a particular operating range of an engine. In the operating range where the feedback control is not carried out, an open loop control is used to control the air-fuel ratio.
In order to enhance the accuracy of the air-fuel ratio in the open loop control range or to suppress a variation in the air-fuel ratio due to the individual difference as small as possible, it was proposed to correct the supply fuel amount at the time of the open loop control using the feedback correction value (Japanese Patent Publication No. 40010/1983)
However, when the control is shifted to the open loop control range in the state that the feedback control is applied as in such conventional control, the problems which follow may arise. In the conventional control, the learning value is altered at every learning time and sequentially optimized, while contribution of the feedback correction decreases. Accordingly, even if the control is shifted to the open loop control range while the feedback correction is applied, no particular effects are expected. Further, variations in the air-fuel ratio due to disorders in the feedback range result directly in the influence on the control of the air-fuel ratio in the open loop control range.